The invention relates to a ladder. In particular, this invention relates to a portable ladder than can be folded for storage and transport, then deployed (unfolded) for use. Such ladders are sometimes used by military or police to scale obstacles encountered in the field.
One ladder of this general type is shown in U.S. Pat. No. 6,318,498 and includes a plurality of ladder segments that are hinged together. Two adjacent segments can freely pivot almost 360 degrees relative to each other, at the hinge joint. Each segment includes one rung and is typically one foot long. When the segments are unfolded and laid flat, the segments and rungs are arranged one after the other along the length of the ladder. The ladder has two side rails, one on each side of the rungs. Each side rail is generally C-shaped and includes three hingedly connected sections that are flat with the rungs when the ladder is folded; that are flat also when the ladder is first unfolded to be deployed; and that pivot or spring out into the C-shape when deployed to form the structural support (side rails) for the rung segments.
Specifically, in each ladder segment, a first or inner section of each side rail is fixed to the rung and extends perpendicular to the length of the rung, in the plane of the rung. A second or middle section of the side rail extends from the outer edge of the inner section, in a direction perpendicular to the inner section, and forms the outermost portion of the side rail. A third rail section or return extends inward from the outer edge of the middle section, in a direction parallel to the inner section. The three rail sections thus form a generally C-shaped box configuration. Springs, or a strap extending across between the two side rails, holds the side rail sections in the deployed condition. The side rails provide structural strength to hold the several ladder segments in position relative to each other without buckling or folding. This is needed because two adjacent ladder segments can freely pivot almost 360 degrees relative to each other, at the hinge joint.
The ladder is folded in an accordion style, with hinge joints between adjacent ladder segments. Specifically, one hinge joint allows for relative pivoting movement of its two connected segments in one direction, while the next hinge joint along the length of the ladder allows for relative pivotal movement of its two connected segments in the opposite direction. This allows the ladder to be folded, accordion style, into a small package that can be transported more easily. But because the hinge joints do not lock, they do not provide any structural strength to hold the ladder in the deployed (extended) condition.
It is sometimes desired to employ a foldable ladder of this type in a horizontal orientation, to enable the user to walk or crawl across it. Because the hinge joints of the prior art ladder do not provide any resistance to bending in either direction, the side rails may not be stiff enough to bear the load.